1. Field of the Invention
This invention relates to a method and apparatus for removing image artifacts including moiré from screen images, particularly images generated by liquid crystal projectors (LCPs). The invention also relates to a program of instructions for implementing various aspects of the image artifact removal technique.
2. Description of the Related Art
An image that contains screens, which are composed of periodic lines or dot patterns, sometimes exhibits artifacts including moiré artifacts when projected or displayed. Moiré artifacts may result whenever two geometrically-regular patterns are superimposed and often manifests itself as a ripple-like pattern in the image representations. Such artifacts degrade image quality and are therefore undesirable.
In the case of a projected image, the image-generating projection device (e.g., an LCP) may employ a technique known as a “keystone correction” which alters the shape of the projected image to compensate for the angle of projection. While the keystone correction improves certain characteristics of the projected image, it has a disadvantage in that it is usually not able to maintain equal spacing between screen line/dot patterns. Consequently, the unequal spacing at different locations in the image results in moire artifacts.
While low pass filtering may be used to remove the moiré artifacts, global application of a low pass filter (LPF) tends to cause blurring in text and non-screen regions in the displayed or projected image. That is, if an LPF is applied uniformly to the image, the degree of filtering required to achieve an acceptable reduction in moiré usually results in an unacceptable reduction in resolution.
Thus, there is a need for an effective image artifact removal technique that smoothes screen regions to remove moire while maintaining sharpness in non-screen regions and that is particularly well suited for projected images.